LOS ANGELES (Map, News) - A $200,000 reward was offered Tuesday for the return of as many as 12 paintings by Marc Chagall, Diego Rivera and other masters that were stolen from a San Fernando Valley home.

The stolen paintings were worth millions of dollars, said Los Angeles police Detective Donald Hrycyk. who did not have an exact value. It could be difficult for thieves to sell the works without proof of their authenticity.

The owners were offering the reward for information leading to the recovery of the artwork and the capture of the crooks.

The paintings disappeared Aug. 23 from the home of an elderly couple. One of the residents, who is bedridden, was home at the time. The other was asleep, so neither was aware of an intruder, police said.

The thief entered the house through a service door and took the paintings from the walls of two adjacent rooms.

"Normally there would be people with the victims at the home," Hrycyk said. "If it is a dangerous stranger that just happens off the street, they were very lucky to pick the time that they did."

Nine pictures by Chagall, Rivera, Hans Hofmann, Chaim Soutine, Arshile Gorky, Emil Nolde, Lyonel Feininger, and Kess van Dongen were stolen, and three others appeared to be missing as well.

The stolen paintings were among a collection amassed over the past 60 years or so.